Electrodeless discharge lamp apparatus



Feb. 16, 1965 w. E. BELL 3,170,086

ELECTRODELESS DISCHARGE LAMP APPARATUS Filed Jan. 26, 1962 OSCILLATORGAS CELL GAS CELL INVENTOR. WILLIAM E. BELL 3,170,086 ELECTRODELESSDISCHARGE LAMP APPARATUS William E. Bell, Palo Alto, Calif., assignor toVarian Associates, Palo Alto, Calif., a corporation of California FiledJan. 26, 1962, Ser. No. 169,040- 4 Claims. (Cl. 315248) The presentinvention relates in general to electrodeless discharge lamps, and moreparticularly to a novel circuit arrangement which permits anelectrodeless discharge to be initiated and maintained by a remote powersource.

High frequency electrodeless discharge lamps have recently beendeveloped for use as efiicient and stable sources of highly resolvedoptical spectral line radiation. Such lamps are disclosed in an articleappearing in Review of Scientific Instruments 32,688 (June 1961), andalso in copending US. Patent application Serial No. 56,412, filedSeptember 16, 1960, now US. Patent 3,109,960, assigned to the sameassignee.

The discharge in such lamps is initiated and maintained through the highfrequency fields of an external radio frequency coil which encircles abulb containing an optically radiating gas or vapor. The impedance presented by such a coil to excitation power depends strongly on thecondition of the discharge and hence varies considerably duringoperation. This presents a difficult problem when, for purposes such aselectrical or thermal isolation, it is desired to excite the coil from aremote power source via a transmission line. For example, when the lampis used to optically pump and monitor a gas cell (containing, forexample, an alkali vapor such as rubidium) which senses and measuresweak magnetic fields, it is desired to remove the magnetic influence ofmaterials and currents in the power source.

It is an object of the present invention to provide electrodelessdischarge lamp apparatus adapted for efficient and stable operation whenexcited from a remote power source. Generally, this is accomplished bythe provision of a novel transmission line termination utilizingmagnetic coupling to a tuned circuit which is at least partiallydecoupled after the lamp discharge is initiated by the interposition ofthe then conducting ionized gas of the discharge.

Various features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnect-ion with the accompanying drawing which illustrates anembodiment of the present invention adapted for use in an instrumentemploying optically pumped gas cells.

Excitation power for lamp bulb 1 is provided by a remote R.F. oscillator2, via a coaxial transmission line 3. The line 3 is terminated by asingle turn, low impedance, untuned loop 4 encircling the bulb 1. Loop 4is magnetically coupled to the tank coil 5 of a floating, high Q,parallel resonant circuit. Coil 5 is preferably wound in two sectionswith the end turns 5' adjacent each other to aid in the establishment ofa high field strength for rapidly initiating the discharge, a featureclaimed in US. Patent 3,109,960. The capacitive reactance of theresonant circuit is preferably provided by a pair of capacitors 6symmetrically located relative to ground in order to minimize leakageradiation. These capacitors may be variable to provide a tuningadjustment or alternatively they may be fixed and provision for tuningmade in the RF. oscillator 2.

In a typical embodiment, the diameter of bulb 1 is one centimeter; theexcitation frequency of oscillator 2 is 140 mc./sec.; the characteristicimpedance of line 3 is 50 ohms; coil 5 consists of six turns of No.24.wire;

United States Patent 0 ice and capacitors 6 each have a nominalcapacitance value 0f 10 .t,uf.

In operation, the single loop 4 coupled to the tuned circuit 5, 6presents a good impedance match to the transmission line 3 so that ahigh intensity alternating magnetic field is developed with minimumpower by the high Q circuit 5, 6 and initiates a discharge inside thebulb 1. Thereafter the conductivity of the discharge serves to partiallydecouple the tuned circuit 5, 6 and the discharge is maintained by theloop 4 which now provides a good match to the load despite variations inlamp impedance, the power being absorbed by the conducting ionized gasinstead of the tuned circuit. The length of line 3 is preferably anintegral number of half-wavelengths at the operating frequency to avoidundesired impedance transformation .to the oscillator of small residualreactances which may be present at the bulb end of the line. Thus,stable, high eificiency operation is achieved for all operatingconditions. For example, in the above described typical embodiment, thedesired operation was obtained at an excitation power level of l-2watts-with a standing wave ratio on the transmission line of between 1:1and 1:3.

In the illustrative embodiment, the lamp bulb 1 radiates opticalresonance radiation of the gas or vapor contained therein through asuitable optical system (including lenses, filters, polarizers, etc.,not shown, as desired) to optically excite or pump a pair of gas cells 7which contain the same gas or vapor. This arrangement which convenientlyprovides oppositely directed light beams through the separate gas cellsmay be used, for example, in a magnetometer of the type described andclaimed in US. patent application Serial No. 62,480, filed October 13,1960, abandoned in favor of Serial No. 250,460, filed January 7, 1963,and assigned to the same assignee, in which the intensity modulation ofthe optical radiation passing through each gas cell 7 is detected andfed back to the opposite cell in the form of an alternating magneticfield which maintains the device in self-oscillation at a frequencydetermined by the intensity of the environmental magnetic field at thecells.

Since many changes could be made in the above construction and manyapparently widely different embodi-.

ments of this invention could be made without departing from the scopethereof, it is intended that all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of the NationalAeronautics and Space Act of 1958, Public Law -568 (72 Stat. 426; 42U.S.C. 2451), as amended.

What is claimed is:

1. An electrodeless discharge lamp apparatus compris ing: a bulbcontaining a discharge substance sealed therein, said substance being inthe gaseous state at glow discharge pressure in operation, said bulbhaving at least a light transmissive portion for transmission of lightout of said bulb; exciting means at least partially disposed externallyto said bulb for exciting an optically radiating discharge therein; saidexciting means including a source of RF. energy, a first terminationcircuit means portion disposed inductively adjacent said bulb andcoupled to said source for generating an alternating R.F. magnetic fieldin said bulb which dissipates sufficient power in the discharge whenexcited to maintain said discharge, and a tuned second terminationcircuit means portion tuned to a frequency having a peaked response atsubstantially a frequency of RF. energy supplied from said source anddisposed inductively adjacent said bulb and initially inductivelycoupled to said first termination 3 circuit means portion for buildingup a high intensity alternating field in said bulb which initiates saiddischarge, and thereafter being at least partially decoupled from saidfirst termination circuit means portion by the conductivity of thedischarge.

2. The apparatus according to claim 1 including an R.F. transmissionline connected in circuit between said source of RF. energy and saidfirst termination circuit means portion for supplying R.F. energy tosaid first termination circuit means portion.

3. Apparatus according to claim 2 wherein said first termination circuitmeans portion consists of single turn coil.

4 4. Apparatus according to claim 2 wherein said transmission line issubstantially an integral number of halfwavelengths long at saidexcitation frequency connecting said source to said first terminationcircuit means portion.

References Cited in the file of this patent UNITED STATES PATENTS1,861,620 Buttolph June 7, 1932 1,919,490 Von Lepel July 25, 19332,913,631 Cuker Nov. 17, 1959 2,975,330 Bloom Mar. 14, 1961

1. AN ELECTRODELESS DISCHARGE LAMP APPARATUS COMPRISING: A BULB CONTAINING A DISCHARGE SUBSTANCE SEALED THEREIN, SAID SUBSTANCE BEING IN THE GASEOUS STATE AT GLOW DISCHARGE PRESSURE IN OPERATION, SAID BULB HAVING AT LEAST A LIGHT TRANSMISSIVE PORTION FOR TRANSMISSION OF LIGHT OUT OF SAID BULB; EXCITING MEANS AT LEAST PARTIALLY DISPOSED EXTERNALLY TO SAID BULB FOR EXCITING AN OPTICALLY RADIATING DISCHARGE THEREIN; SAID EXCITING MEANS INCLUDING A SOURCE OF R.F. ENERGY, A FIRST TERMINATION CIRCUIT MEANS PORTION DISPOSED INDUCTIVELY ADJACENT SAID BULB AND COUPLED TO SAID SOUCE FOR GENERATING AN ALTERNATING R.F. MAGNETIC FIELD IN SAID BULB WHICH DISSIPATES SUFFICIENT POWER IN THE DISCHARGE WHEN EXCITED TO MAINTAIN SAID DISCHARGE, AND A TURNED SECOND TERMINATION CIRCUIT MEANS PORTION TUNED TO A FREQUENCY HAVING A PEAKED RESPONSE AT SUBSTANTIALLY A FREQUENCY OF R.F. ENERGY SUPPLIED FROM SAID SOURCE AND DISPOSED INDUCTIVELY ADJACENT SAID BULB AND INITIALLY INDUCTIVELY COUPLED TO SAID FIRST TERMINATION CIRCUIT MEANS PORTION FOR BUILDING UP A HIGH INTENSITY ALTERNATING FIELD IN SAID BULB WHICH INITIATES SAID DISCHARGE, AND THEREAFTER BEING AT LEAST PARTIALLY DECOUPLED FROM SAID FIRST TERMINATION CIRCUIT MEANS PORTION BY THE CONDUCTIVITY OF THE DISCHARGE. 